Within the field of electrochemistry, there is a well-known type of an electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas and caustic soda, viz., sodium hydroxide, are produced by passing an electric current through a concentrated salt (brine) solution containing sodium chloride and water. A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the caustic soda.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION.RTM. manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g, asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode. One of the unwanted by-products present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
Fairly recently, attention has been directed in chlor-alkali cell technology to various forms of oxygen (air) cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed to operate chlor-alkali cells. Estimates indicate that there is a theoretical savings of about 25 percent of the total electrical energy required to operate chlor-alkali cells provided that the formation of hydrogen at the cathode can be prevented. In other words, about 25 percent of the electrical energy employed in a chlor-alkali cell is used to form hydrogen at the cathode. Hence, the prevention of hydrogen formation by forming hydroxide at the cathode results in significant savings in the cost of electrical power. This is the major benefit of and purpose for oxygen (air) cathodes. Such cathodes, being in contact with the electrolyte caustic soda, are subjected to the corrosive action thereof. Additionally, there are internal stresses and forces produced by the very reactions occurring at the cathode which tend to cause deterioration, break up of the active layer and bleed through of liquid in the wetproofing (backing) layer of such electrodes.
One known form of oxygen (air) cathode involves use of an active cathode layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed (enhanced) using precious metal catalyst materials, such as silver platinum, etc. Unfortunately, however, the pores of such active carbon particles may become flooded by the caustic soda thereby significantly reducing their ability to eliminate the formation of hydrogen at the cathode and resulting in decreased operating efficiency. Various attempts have been made to solve this wettability problem, e.g., by providing a backing layer which is hydrophobic to reduce the likelihood of wetting or flooding of the carbon particles in the active layer by the catholyte liquor. Various forms of polytetrafluoroethylene (PTFE) have been utilized for this purpose. With the use of PTFE, however, comes the problem of reduced electrical conductivity in the cathode active layer in as much as PTFE, per se, is nonconductive. Some oxygen (air) cathodes contain PTFE in both the active layer and in a backing sheet laminated thereto. Such PTFE has been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the desired layer. Thus it can be seen that the development of corrosion-resistant oxygen (air) cathodes of improved durability for use in conjunction with chlor-alkali cells is an overall objective in the field of electrochemistry.